This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-310144, filed Oct. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to an optical device for optically processing information by light beam scanning, and more particularly, to a magneto-optical pickup for detecting magneto-optical signals.
Recently developed magneto-optical pickups are designed so that a plurality of functions and elements are integrated to ensure smaller sizes and lower costs.
Jpn. Pat. Appln. KOKAI Publication No. 8-329544 discloses a small-sized magneto-optical pickup, which is schematically shown in FIG. 12.
In FIG. 12, a beam of laser light emitted from a semiconductor laser 17 is transmitted through a polarizing prism 23 and converged on an information storage medium 26 by an objective lens 25.
The beam of laser light including information, which is reflected by the information storage medium 26, is split into two by a surface 23a of the polarizing prism 23 in accordance with the polarization. Light components transmitted through the surface 23a are diffracted by a holographic diffraction element 22 and then detected as focus and tracking error signals by light receiving elements 18 and 19 on a substrate 16.
On the other hand, a beam of light components reflected by the surface 23a is deflected by a mirror surface 23b, and then split into two by a prism-type analyzer 24, which includes a plurality of prisms and a polarizing film, located on the substrate 16. The split beams are detected as magneto-optical signals by a light receiving element group 20.
The light receiving elements 18 and 19 and the light receiving element group 20 are all formed on the same substrate 16, and the laser light source 17 is also mounted on the substrate 16. The substrate 16, along with the prism-type analyzer 24, is sealed mainly in a small-sized resin package.
Jpn. Pat. Appln. KOKAI Publication No. 8-329544 also teaches a method for detecting magneto-optical pickup signals by using a polarizing diffraction element in stead of the prism-type analyzer 24, with a beam of light including information split into three beams of 0th and 1st order diffracted light.
Further, Jpn. Pat. Appln. KOKAI Publication No. 10-143934 disclosed an improved magneto-optical pickup based on an additionally developed technique, which is schematically shown in FIG. 13.
In FIG. 13, a laser beam emitted from a laser diode 3 is transmitted through a polarizing prism 9 and converged on an information storage medium 13 by a objective lens 12.
The laser beam reflected by the information storage medium 13 is split into two by a surface 9a of the polarizing prism 9 in accordance with the polarization. Light components transmitted through the surface 9a are diffracted by a holographic diffraction element 8 and detected as error signals, such as focus and tracking error signals, by light receiving elements 4 and 5 on a substrate 2.
On the other hand, a beam of light components reflected by the surface 9a is deflected by a mirror surface 9b, and then split into two beams in accordance with the polarization by a Wollaston prism 10, which is united with the polarizing prism 9. The split beams are detected as magneto-optical signals by a light receiving element group 6.
The light receiving elements 4 and 5 and the light receiving element group 6 are all formed on the same substrate 2, and the laser light source 3 is also mounted on the substrate 2. The substrate 2 is sealed mainly in a small-sized resin package.
Using the prism-type analyzer for the separation of the magneto-optical signals, the magneto-optical pickup shown in FIG. 12 includes a very large number of components. Further, each component requires high working accuracy, and its assembly also requires high accuracy.
In particular, mounting the prism-type analyzer itself on the substrate requires very high accuracy, and naturally, mounting the polarizing prism for guiding light to the analyzer also requires high accuracy. If the mounting accuracy of the analyzer 24 is unsatisfactory, therefore, adjustment cannot be carried out when the polarizing prism 23 is mounted, in some cases. If the mounted analyzer is concluded to be defective by inspection after assembly, therefore, the semiconductor laser, light receiving element substrate, and package, which are rather expensive, must possibly be scrapped.
Use of the polarizing diffraction element for separation of the magneto-optical signals also requires high working and mounting accuracies. Further, the polarizing diffraction element itself is very expensive. Since a light beam is split at angles by the diffraction element, in particular, the distances between the diffraction element and the light receiving element group must be adjusted and maintained with high accuracy. Partly because the package is formed mainly of resin, moreover, the resulting pickup is susceptible to change of the environmental conditions.
The magneto-optical pickup shown in FIG. 13 entails high manufacturing cost, since it uses the very expensive Wollaston prism for the separation of the magneto-optical signals. This pickup, like the foregoing magneto-optical pickup, moreover, requires high mounting accuracy.
Since the Wollaston prism deflects beams in ordinary and extraordinary rays in different directions, in particular, high mounting accuracy is needed to focus spots correctly on the light receiving elements. Further, fluctuations of the distance between the Wollaston prism and the light receiving element substrate prevent the spots from being focused correctly on the light receiving elements. Since the package is formed mainly of resin, this magneto-optical pickup is susceptible to change of the environmental conditions. In order to secure a long distance of separation between the beams in ordinary and extraordinary rays, furthermore, the space between the Wollaston prism and the light receiving element substrate should be adjusted to a certain dimension. Accordingly, the size of the pickup can be reduced only limitedly.
Thus, according to the conventional magneto-optical pickups, a lot of components or expensive parts must be assembled with very high accuracy.
The object of the present invention is to provide a magneto-optical pickup, which requires relatively low mounting accuracy and is small-sized and low-priced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the instrumentalities and combinations particularly pointed out hereinafter.